Basic suspension, its preparation and process for paper deacidification

ABSTRACT

Basic suspension wherein oxides hydroxides, carbonates, or their mixture, in the form of particles having determined dimensions are suspended in the appropriated solvents, are described. Processes for the preparation of the above said suspensions and of the suspended particles are also described together with the uses of the suspensions in processes for paper deacidification and/or porous materials consolidation.

FIELD OF THE INVENTION

The present inventions refers to basic suspensions which, thanks totheir physico-chemical properties, can annul the acidity present inpapers, which depends on the paper production processes, on theenvironment pollution, and on the usage of acid inks as, for example, inthe modern books.

STATE OF THE ART

It is well known how the presence of acidity in paper is the mainresponsible of its degradation with consequent loss of a lot ofimportant information and in particular of historically importantdocuments.

The acidity, which is present particularly in modem paper, forms duringthe paper manufacture, it is also a consequence of the use of acid inks(which are very common especially in the last centuries) or because ofthe adsorption of acid pollutants which are present in the air.

Many different techniques and products have been studied or developed inorder to eliminate acidity from paper and published documents (paperde-acidification). Unfortunately, up to now, the problem is far to besatisfactorily solved, and the specialists are intensively looking fornew products capable of annulling the acidity present in paper.

Calcium, magnesium, and barium hydroxide aqueous solutions have beenwidely used for many decades, but, unfortunately, they had often inducednot desirable side effects, because of their strong alkaline conditions,with subsequent depolymerization of cellulose.

Non aqueous deacidification processes have also been proposed. In SmithR. D., Mass Deacidification at the Public Archives of Canada inConservation of Library and Archive Materials and the Graphic Arts,Butterworth Ed., London, 1987 the use of magnesium alkoxidedeacidification solutions (Wei T'o method) is described; another methodwidely used is the Bookkeeper method which is based on the applicationof suspended Mgo that, subsequently converted in magnesium hydroxide,forms the alkaline reservoir. Unfortunately, these methods usechlorofluorocarbon (CFC) solvents. Alternatively, some solvent mixtureshave been recently experimented, as thrielyne (90%) and methanol, butall of them are harmful.

Despite of its scarce usage, calcium hydroxide is an excellentdeacidifying agent. Calcium hydroxide is physico-chemically friendly tomost of papers and, once converted to calcium carbonate, worksefficiently as alkaline reservoir. In IT 1.286.868 (in the name of thesame Applicant) it is reported that suspensions of calcium hydroxidecould be used to consolidate mural paintings and to deacidify paper.However, the calcium hydroxide suspensions in organic solvent describedin this document were obtained from lime and the particle sizedistribution was quite broad and larger than several micrometers.Smaller particles were around 0.5 microns, 80% were greater than 1micron. This originated two problems: firstly, kinetic stability wasgood but not excellent for some application procedures, for examplespraying; secondly, because of particle sizes, a white glaze (or spots)on the treated surfaces could form.

Large calcium hydroxide particles showed a poor adhesions andpenetrability into the cellulosic fibres.

Deacidifying agents must ensure good adhesion with fibres, so that thealkaline reservoir acts efficiently. Therefore, according tophysico-chemical features of cellulose, particles with a surface chargedensity are preferred. Both these requirements are ensured by the use ofparticles smaller than micron size.

SUMMARY OF THE INVENTION

The present invention refers to basic suspension wherein the suspendedparticles have determined dimensions, moreover the invention refers toprocess for the preparation of the suspensions and of the suspendedparticles and to the use of the suspension in processes of paperdeacidification and porous materials consolidation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention makes it available basic suspension which allowthe overcoming of the problems connected with paper acidity.

The suspension according to the inventions are preferably suspensions ofbasic oxides or hydroxides (which are capable of forming carbonates whenin contact with the CO₂ of the air or to form, once adsorbed on thetreated material, a reservoir of basicity stable for long periods oftime) or basic carbonates or their mixture.

Particularly preferred according to the invention are the oxides,hydroxides or carbonates of alkali or earth-alkali metals or also otheranalogous compounds having basic properties.

Examples of compounds suitable for the suspensions according to theinvention are: Li₂O, Na₂O, K₂O, MgO, CaO, SnO, SnO₂, PbO, Pb₂O Pb₂O₃,BiO, Bi₂O₃, Sb₂O₃, LiOH, NaOH, KOH, Mg(OH)₂, Ca(OH)₂, Al(OH)₃, Sn(OH)₂,Sn(OH)₄, Pb(OH)₂, Bi(OH)₃, Sb(OH), Li₂CO₃, Na₂CO₃ decahydro, Na₂CO₃,K₂CO₃, MgCO₃, CaCO₃, PbCO₃, anhydro and basic, Bi₂O₂CO₃ or theirmixtures.

Particularly preferred according to the invention are the suspensions ofMg(OH)₂, Ca(OH)₂, Sn(OH)₂.

The preferred suspending solvents are chosen in the group consisting of:water, ethyl ether, acetone, alcohols, and their mixtures.

Among the alcohols particularly preferred are: methanol, ethanol,1-propanol, 2-propanol, butanol, pentanol, and their mixtures.

Particularly preferred are suspension of the Ca(OH)₂ hydroxide in 1%(w/w) in water and 99% in 1-propanol (or 2-propanol).

The dimensions of the hydroxide particles in suspension play also acrucial role in the suspensions according to the invention. In fact,such particles must be capable of entering the fibres of the materialsto treat, linking with them, without covering effect over the writtentest since this would produce a not desirable aesthetically effect.Large particles need longer time to penetrate in depth, but the fastvolatillsation of used organic solvents does not usually allow it.Therefore, a strict control of particle size is essential before theapplication.

Particularly preferred are suspensions according to the inventionwherein the suspended particles have dimension comprised between 10 nmup to 21 μm, more preferably between 50 nm up to 500 nm.

The suspensions according to the invention have a concentration ofsuspended particles in the solvent preferably comprised between: 0.1-50g/L more preferably between 1-25 g/L.

Especially preferred is a concentration of: 10 g/L.

The suspensions according to the invention can be obtained by vigorousstirring of the particles to be suspended in the appropriate solvent (orsolvents mixture), if necessary with the aid of a sonicator orultra-sonicator or with the aid of a homogeniser.

The particles to be suspended, having the desired measures, can beobtained by high temperature homogeneous phase reaction, heterogeneousphase reaction (slaking of oxides), and by mixing two water in oilmicroemulsion obtaining a low solubility salt in nanosized aqueous core.

The first process consists in reacting solutions of the appropriatereagents (for example a solution of an hydroxide as NaOH and a solutionof a chloride as CaCl₂) at high temperature (about 90° C.) understirring.

The solutions to be reacted can be obtained by solving the reagents in asolvent chosen in the group consisting of: water, a solution of diols ora mixture water/diols (the reagents can be solved both in the same oreach one in a different solvent 15′ chosen from the above said group).

The preferred diols are C₁alkane diols as, for example, 1,2 ethandiol or1,2 propandiol.

The suspension obtained after reaction at high temperature andcontaining the formed particles is cooled down to room temperature underN₂, the supernatant solution is eliminated and the suspension is washedto eliminate secondary reaction products and concentrated in vacuum toform a paste which can be suspended again in the desired solvent amount.

According to the slaking of oxides process the oxide is slaked in purewater, the obtained hydroxide paste is mixed with an alcohol undervigorous stirring and sonicated or homogenised.

Alternatively, according to known techniques, two water in oilmicroemulsions are prepared wherein the aqueous phase containing salts,when the two microemulsions are mixed precipitation takes place whereinthe nanosized dimensions of the aqueous drops impose limited growth ofparticles.

Given the above said, examples of suspensions according to the inventionare reported hereinafter in order to better illustrate the invention:

EXAMPLE 1

Calcium hydroxide micro-particles have been obtained by slaking CaOpowder in pure water, at room temperature: CaO powder 10 g Water 30 g

Successively, suspensions have been prepared by mixing the calciumhydroxide paste with alcohol, under vigorous stirring, and have beensonicated for 30 minutes with an ultra-sonicator for further reducingthe particle size and obtaining a complete dispersion of the calciumhydroxide agglomerates. A homogeniser system, for preparation of somehydroxide dispersions, can also be used.

EXAMPLE 2

Calcium hydroxide micro-particles have been obtained by slaking CaOpowder in water/alcohol mixtures, according to the followingproportions: CaO powder 1 g Water 5 g 1-propanol 25 mL

The metal oxide/water/alcohol mixture, after 30 minutes of sonication orvigorous stirring has been left to rest at least B hours. Suspensionshave been prepared by mixing the calcium hydroxide paste, afterfiltration, with alcohol, under vigorous stirring. Suspensions have beensonicated for 30 minutes with an ultra-sonicator for further reducingthe particle size and obtaining a complete dispersion of the calciumhydroxide agglomerates. A homogeniser system, for preparation of somehydroxide dispersions, was also necessary.

EXAMPLE 3

Calcium hydroxide nanoparticles, from homogeneous phase reaction, havebeen achieved by mixing a NaOH aqueous solution with a CaCl₂ aqueoussolution. They were previously heated up to a selected temperature(around 90° C.), under continuous stirring and keeping the temperatureof the mixture constant within ±1° C.; the supersaturating degree waskept in the range 2-10. Aqueous Ca(OH)₂ suspension was allowed graduallyreaching the room temperature under a nitrogen atmosphere to avoid theM(OH)₂ carbonation. The supernatant solution was discarded and theremaining suspension was washed five times with water to reduce the NaClconcentration below. 10⁻⁶ M. Each time, the dilution ratio between theconcentrated suspension and the washing solution was about 1:10. Thecomplete removal of NaCl from the suspension was controlled by the AgNO₃test. The suspension was then concentrated in vacuum at 40° C. up to aweight ratio M(OH)₂/water of 0.8, that is the same of the standardslaked lime paste. Dispersions have been prepared by mixing 10 g of thecalcium hydroxide paste with 1 L of alcohol, under vigorous stirring,and/or by using a homogeniser.

EXAMPLE 4

Calcium hydroxide nanoparticles, from homogeneous phase reaction, havebeen obtained by mixing a NaOH aqueous solution with a CaCl₂ diolsolution (i.e. 1,2 ethandiol or 1,2 propandiol). Diol solution waspreviously heated up to a selected temperature (around 150° C.), undercontinuous stirring and keeping the temperature of the mixture constantwithin ±1° C.; the supersaturating degree was in the range 2-10. TheNaOH aqueous solution was added dropwise to dial solution keeping thetemperature around 150° C. The Ca(OH)₂ water/diol suspension was allowedgradually reaching the room temperature under a nitrogen atmosphere toavoid the M(OH)₂ carbonation. The supernatant solution was discarded andthe remaining suspension was washed five times with water to reduce theNaCl concentration below 10⁻⁶ M. Each time, the dilution ratio betweenthe concentrated suspension and the washing solution was about 1:10. Thecomplete removal of NaCl from the suspension was controlled by the AgNO₃test. Then, water/diol suspension was washed several times with2-propanol to remove diol solvents. Successively, solutions wereconcentrated in vacuum at 40° C. up to a weight ratio M(OH)₂/water of0.8, that is the same of the standard slaked lime paste. Dispersionshave been prepared by mixing log of the calcium hydroxide paste with 1 Lof alcohol, under vigorous stirring, and/or by using a homogeniser.

EXAMPLE 5

Two water in oil microemulsions have been obtained.

C₁₂E₄ surfactant solutions in cyclohexane have been prepared and used asoil phase in the microemulsion. These solutions have been mixed withNaOH (0.1M) and CaCl₂ (0.05M) aqueous solution.

Microemulsion (water in oil) composition is: C₁₂E₄ 0.15 g (0.2 M)Cyclohexane 1.15 g NaOH (0.1M) and CaCl₂ (0.05M) aqueous solution 25 μL(w = 2)

Water/surfactants molar ratio ranging (molar ratio w=water/surfactant)from 1 to 5. Aqueous solution was added to oil phase/surfactant solutionby a microsyringe. The preparation was held at 15° C. Similar isotropic,transparent, and stable systems, were obtained in a wider temperaturerange, 15-25° C. Microemulsions have been equilibrated at 15° C. for 10days. Crystalization processes took place within the confined volume ofthe microdroplets. Synthesized particles, after filtration, washing, anddrying have been dispersed in nonaqueous solvents, by vigorous stirringand/or with the aid of a homogeniser.

EXAMPLE 6

Solution of Igepal CO 520 in cyclohexane (0.15M) has been prepared andmixed, respectively, with an aqueous solution of NaOH (0.5M) and with anaqueous solution of CaCl₂ (0.25M).

Microemulsion (water in oil) composition is: Igepal CO 520 solution(0.15M) 1 g NaOH (0.5M) and CaCl₂ (0.25M)aqueous solution 25 μL (w = 2)

Water/surfactants molar ratio ranging (molar ratio w=water/surfactant)from 1 to 11.

Aqueous solution was added to oil phase/surfactant solution by amicrosyringe. The preparation was held at environmental (25° C.).Analogous systems, isotropic, transparent, and stable was obtained in awider temperature range, 15-25° C. Microemulsions have been equilibratedat 25° C. for a week.

Synthesized particles, after filtration, washing, and drying have beendispersed in nonaqueous solvents, by vigorous stirring and/or with theaid of a homogeniser.

Procedures for Application

The physicochemical properties of the prepared dispersions make themvery attractive for different application procedures in paperdeacidificatio processes, such as:

-   -   spraying,    -   immersing,    -   brushing,    -   mass-deacidification process.

Small particle dimensions allows using a sprayer without risks of holeocclusion. The spraying method is preferred.

The complete immersion of paper sheet is possible also for longerperiod. Aqueous deacidification methods, because of the strongaggressive effect of high pH, do not allow a long contact with papersheet, commonly up to 20 minutes. This produces only a poordeacidification effect.

Moreover the basic suspensions according to the invention can be usedalso for the consolidation of porous materials as for example stones,plasters building materials and similar.

1) (Cancelled) 2) (Cancelled) 3) (Cancelled) 4) (Cancelled) 5)(Cancelled) 6) (Cancelled) 7) (Cancelled) 8) (Cancelled) 9) (Cancelled)10) (Cancelled) 11) (Cancelled) 12) Basic suspension wherein thesuspended particles have dimensions comprised between 10 nm up to 2 μm.13) Basic suspensions according to claim 12 wherein the suspendedparticles have dimensions comprised between 50 nm up to 500 nm. 14)Basic suspensions according to claim 12 wherein the suspended particlesare basic oxides or hydroxides or carbonates or their mixture. 15) Basicsuspensions according to claim 14 wherein the suspended basic oxides arechosen in the group consisting of: Li₂O, Na₂O, K₂O, MgO, CaO, SnO, SnO₂,PbO, Pb₂O Pb₂O₃, BiO, Bi₂O₃, Sb₂O₃ or their mixtures. 16) Basicsuspensions according to claim 14 wherein the suspended basic hydroxidesare chosen in the group consisting of: LiOH, NaOH, KOH, Mg(OH)₂,Ca(OH)₂, Al(OH)₃, Sn(OH)₂, Sn(OH)₄, Pb(OH)₂, Bi(OH)₃, Sb(OH)₃ or theirmixtures. 17) Basic suspensions according to claim 14 wherein thesuspended basic carbonates are chosen in the group consisting of:Li₂CO₃, Na₂CO₃ deca-hydro, Na₂CO₃, K₂CO₃, MgCO₃, CaCO₃, PbCO₃, anhydroand basic, Bi₂O₂CO₃ or their mixtures. 18) Basic suspensions accordingto claim 12 wherein the solvents are chosen in the group consisting of:water, ethyl ether, acetone and alcohols and their mixtures. 19) Basicsuspension according to claim 18 wherein the alcohols are chosen in thegroup consisting of: methanol, ethanol, 1-propanol, 2-propanol, butanol,pentanol, and their mixtures. 20) Basic suspension according to claim 12consisting of: Ca(OH)₂ hydroxide particles suspended in 1% water and 99%1-propanol or 2-propanol (w/w). 21) Process for the preparation of thesuspensions according to claim 11 by vigorous stirring of the particlesto be suspended in the appropriate solvent (or solvents mixture), ifnecessary with the aid of a sonicator or ultra-sonicator or with the aidof a homogeniser. 22) Process according to claim 21 wherein theparticles to be suspended are obtained by high temperature homogeneousphase reaction. 23) Process according to claim 21 wherein the particlesto be suspended are obtained by slaking of the oxides. 24) Processaccording to claim 21 wherein the particles to be suspended are obtainedby mixing two water in oil microemulsions, giving a low solubility saltin nanosized aqueous core. 25) Process for paper deacidification whereinsuspensions according to claim 12 are applied to the papers to bedeacidified by: spraying, immersing, brushing, or bymass-deacidification process. 26) Processes for the consolidation ofporous materials wherein suspensions according to claim 12 are appliedto said porous materials.